Lithographic printing plates contain areas that accept ink, areas that repel ink, and, ideally, no areas of intermediate properties. The illusion that some areas of a printed image are of intermediate density is created by the viewer's inability to resolve tiny dots of maximum density and background areas of minimum density that separate them. Such images are referred to as halftone images.
Halftone images are typically produced on lithographic printing plates by contact exposure through a high contrast photographic transparency--often referred to as a lith type photographic element. To be useful for the exposure of lithographic printing plates the photographic element must have a contrast of at least 10 (hereinafter designated by reference to high contrast) and more typically near or above 20. Over and above contrast requirements, however, it is important that a lith type photographic element applied to halftone imaging exhibit the characteristic of producing high dot quality. That is, the dots must be well resolved--e.g., with sharp as opposed to soft or ragged edges.
The use of hydrazines in negative working surface latent image forming silver halide emulsions and photographic elements to increase speed and contrast is taught by Trivelli et al U.S. Pat. No. 2,419,975. Increased contrast attributable to hydrazines in negative working surface latent image forming silver halide emulsions is believed to result from the promotion of infectious development.
The hydrazines preferred for their higher effectiveness in increasing contrast are arylhydrazides. The acyl moiety of arylhydrazides increases activity while the aryl moiety acts to increase stability. The following are illustrative of specific arylhydrazides employed with negative working surface latent image forming silver halide emulsions primarily to increase contrast:
P-1 Takada et al U.S. Pat. No. 4,168,977 PA0 P-2 Takada et al U.S. Pat. No. 4,224,401 PA0 P-3 Okutsu et al U.S. Pat. No. 4,221,857 PA0 P-4 Mifune et al U.S. Pat. No. 4,243,739 PA0 P-5 Mifune et al U.S. Pat. No. 4,272,614 PA0 P-6 Mifune et al U.S. Pat. No. 4,323,643. PA0 P-7 Nothnagle U.S. Pat. No. 4,269,929. PA0 P-8 Ives U.S. Pat. No. 2,563,785. PA0 P-9 Whitmore U.S. Pat. No. 3,227,552 PA0 P-10 Leone et al U.S. Pat. No. 4,030,925 PA0 P-11 Leone et al U.S. Pat. No. 4,031,127 PA0 P-12 Leone et al U.S. Pat. No. 4,080,207 PA0 P-13 Tsujino et al U.S. Pat. No. 4,245,037 PA0 P-14 Hirano et al U.S. Pat. No. 4,255,511 PA0 P-15 Adachi et al U.S. Pat. No. 4,266,013 PA0 P-16 Leone U.S. Pat. No. 4,276,364
While the arylhydrazide is preferably incorporated directly in a photographic element, it can be incorporated in a processing solution for the element. A preferred processing solution is disclosed in the following patent:
In surface latent image forming silver halide emulsons the grains which are exposed to light are rendered developable while grains which are not exposed to light are not intended to be developed. Nevertheless, some of these unexposed grains develop spontaneously. In fulltone imaging the spontaneously developing grains raise minimum density more or less uniformly. Such minimum density levels are referred to as fog and, so long as they remain low, are not objectionable.
Pepper fog differs from ordinary fog in that it takes the form of small, maximum density areas randomly distributed on a substantially uniform minimum density background. When a photographic element exhibiting pepper fog is viewed under magnification, the impression to the viewer is often that the magnified field of view has been sprinkled with grains of pepper. It has been long recognized that arylhydrazides produce pepper fog, and, until very recently, this has interfered with the commercial use of arylhydrazides in halftone imaging. An illustration of severe pepper fog appears in FIG. 1. FIG. 2 is an illustration of pepper fog at its highest acceptable level. Each of FIGS. 1 and 2 is an actual photograph of a field of view enlarged 20.times..
In addition to their use to increase contrast in negative working silver halide emulsions arylhydrazides have been employed extensively as nucleating agents in direct positive silver halide emulsions. When employed as a nucleating agent the function of the arylhydrazide is to replace uniform light exposure or aerial oxidation as a technique for selectively accelerating development of unexposed internal latent image forming silver halide grains. This increases maximum density without increasing minimum density of the direct positive image. The use of a hydrazine as a nucleating agent was first taught by
Exemplary arylhydrazides employed as nucleating agents in internal latent image forming direct positive emulsions are illustrated by the following:
RD-1 Research Disclosure, Vol. 151, November 1976, Item 15162 PA1 RD-2 Sidhu et al Research Disclosure, Vol. 176, December 1978, Item 17626 PA1 RD-3 Research Disclosure, Vol. 235, November 1983, Item 23510.
Research Disclosure is published by Kenneth Mason Publications, Ltd., The Old Harbourmaster's, 8 North Street, Emsworth, Hampshire P010 7DD, England.
Parton et al U.S. Pat. No. 4,459,347 discloses oxythioamido substituted arylhydrazides to be useful in increasing the speed of negative working surface latent image silver halide emulsions and as nucleating agents in direct positive internal latent image forming silver halide emulsions.
Hess et al U.S. Ser. No. 493,480, filed May 11, 1983, commonly assigned, discloses sulfinic acid radical substituted arylhydrazides to be effective for (i) increasing contrast of negative working surface latent image forming silver halide emulsions, (ii) nucleating direct positive internal latent image forming silver halide emulsions, and (iii) increasing the speed of negative working emulsions of less than high contrast when adsorbed to grain surfaces. For applications (i) and (ii) ballasting groups are disclosed, such as alkyl, alkoxy, amido, carbamoyl, oxyamido, carbamoyloxy, carboxy, oxycarbamoyl, phenyl, alkylphenyl, phenoxy, and alkylphenoxy groups as well as groups which are combinations thereof. For applications (ii) and (iii) the use an adsorption promoting moiety, such as an oxythioamido substituent, is disclosed.
A patent literature summary of arylhydrazides employed to increase contrast in negative working silver halide emulsions and to perform nucleation in direct positive silver halide emulsions, which includes a discussion of the mechanism of activity in each system is provided by the following publication:
An oxycarbamido substituent of an arylhydrazide is disclosed.